clark



G. W. CLARK July 16, 1963 SYSTEM FOR PREVENTING OVERHEATING OF A STEAMTURBINE Filed Dec. 26, 1957 FIG.I

INVENTOR. GORDON W. CLARK HIS ATTORNGY United States Patent ()fi ice-3,097,487. Patented July 16, 1963 3,097,487 SYSTEM FGR PREVENTINGOVERHEATING OF A STEAM TURBINE Gordon W. Clark, Burnt Hiils, N.Y.,assignor to General Electric Company, a corporation of New York FiledDec. 26, B57, Ser. No. 705,272 7 Claims. (Ci. 60-73) This inventionrelates to elastic fluid turbine powerplants, particularly to a bypassarrangement for a large reheat steam turbine to prevent overheating ofthe high pressure turbine when steam is trapped therein by suddenclosing of the control valves.

In a conventional reheat steam turbine powerplant comprising a highpressure turbine, reheater, and lower pressure turbine, the flow to thehigh pressure turbine is controlled by main control valves and the steamflow from the reheater to the lower pressure turbine is controlled by anintercept valve. The main control and intercept valves are moved to aclosed position by a main speed governor and pre-emergency governorrespectively when the turbine reaches a predetermined overspeedcondition or by suitable tripping mechanism when there is a failure inthe generator electrical system, the steam supply system, or failure ofsome essential service to the turbine, etc., which will be referred toherein as system failures. The sudden closing of the main control andintercept valves results in the entrapment of a substantial mass ofsteam at high temperature and pressure in the high pressure turbine,reheater and interconnecting conduits. This trapped steam, if allowed toremain, is violently agitated by the high speed rotor, and the fluidfriction raises the steam temperature well above the normal operatingsteam temperature. Because the turbine rotor and related parts aredesigned to operate normally very close to the maximum safe temperature,this increased temperature condition could cause a serious reduction inthe strength of the turbine components.

With the tendency to build turbines which operate at higher and highertemperatures and pressures, there is an increased need to evacuate thetrapped steam to prevent the highly undesirable temperature riseresulting from these higher operating conditions.

Accordingly, it is an object of this invention to prevent overheating ofthe high pressure section of a reheat turbine when steam is trappedtherein by sudden closing of the main control and intercept valves.

A further object is to provide a system of the type described in which,when the main control and intercept valves are closed, the steam trappedin the high pressure turbine is automatically vented to the condenser ina manner so as to remove the fluid friction heat generated in the highpressure turbine.

A still further object is to provide a bypass system of the typedisclosed which is controlled in accordance with the closing movement ofthe main control valves.

Other objects and advantages will be apparent from the followingdescription taken in connection with the accompanying drawing, in whichFIG. 1 represents diagrammatically a large high temperature compoundreheat steam turbine embodying my invention; and FIG. 2 illustrates aminor modification of one element of the system.

Generally stated, the invention is practiced by automatically ventingthe high pressure turbine casing to the condenser in such a manner thatthe trapped steam flows backwardly through the high pressure turbinewhen the main control and intercept valves are closed. The ventingsystem is controlled by valve means actuated simultaneously with theclosing of the main steam control valves, caused by turbine overspeed orother system failures.

Referring now more particularly to FIG. 1 of the drawing, the inventionis illustrated as applied to a steam turbine powerplant having a highpressure turbine section 1, and at least one lower pressure turbinesection 2. These may be on different shafts, or may be coupled togetheron a common axis as shown in the drawing. The turbine 2 may exhaust intostill lower pressure turbines, or directly into the condenser 3. Thecondensate is returned by the boiler feed pump 4 to the steam generator5, which is shown as having primary steam generating coils 5a and areheater section 5b.

The path of the turbine motive fluid is from the boiler feed pump 4 tothe primary steam coils 5a, then through the main control valve 7, tothe high pressure turbine 1, and back to the reheater 5b. Steamdischarged from the reheater 5b passes through the intercept valve 10,and then to the inlet of the lower pressure turbine 2. This simplediagrammatic showing, of course, does not include many conventionalelements of steam powerplants, such as feedwater heaters, lubricatingsystems, packing control system, and numerous minor details of theturbine control system, the arrangement of which will be understood bythose familiar with steam powerplant design.

The steam controlling system comprises a main speed governor 12 drivenby turbine shaft 13 through suitable gearing 14, which controls the maininlet valve 7 through rod 12a and a suitable hydraulic force-multiplyingrelay 15. This governor is designed to begin closing valve 7 whenturbine 1 exceeds of rated speed, and fully closes the valve atapproximately of rated speed.

The reheat steam control mechanism comprises a preemergency governor '16driven by turbine shaft. 13 to control intercept valve 10 through rod16a, relay 16b and lever 16c. Intercept valve 10 begins closing at 101%of rated speed and is fully closed at 105% of rated speed.

The foregoing is merely a diagrammatic showing of a conventionalarrangement for controlling the main inlet and intercept valves inaccordance with the speed of the turbines. It does not include otherconventional elements such as the emergency governor whichinstantaneously closes suitable stop valves (not shown) to completelyshut down the turbine when it exceeds a predetermined emergencyoverspeed condition. An embodiment of mechanism of this type isdisclosed more particularly in US. Patent 2,747,373, issued to M. A.Eggenberger, and assigned to the assignee of the present invention. Forthe present purpose, it is suflicient to note that the function ofrelays 15, 16b is to close main valve 7 and intercept valve 10,respectively, when the oil supply thereto is cut off by an emergencytripping mechanism (not shown) as a result of any one of several typesof system failures requiring that the turbine unit be shut down.

From the foregoing, it can be seen that when the main control valve 7and intercept valve 10 close, a substantial volume of steam at a highpressure (perhaps 600 p.s.i.a.) and temperature (perhaps 900 F.) istrapped in the high pressure turbine 1, conduit 11, reheater 5b, andconduit 8 extending between the reheater 5b and intercept valve 10. Thisdense steam trapped in turbine casing 1a is violently agitated by theaction of rotor 1b and the resulting fluid friction increases the steamtemperature significantly, which could seriously overheat the turbinecomponents. If this friction heating of the steam were permited, itwould quickly result in reducing the strength of the rotor and statormembers, to an extent making failure quite possible.

In accordance with my invention, I provide a. system for venting thetrapped steam from conduit 8, reheater 5b and conduit 11 backwardlythrough the high pressure turbine 1 to the condenser 3 when the controland intercept valves are closed thereby removing the friction heatgenerated in turbine 1. As will be more fully discussed hereinafter, thetrapped steam is exhausted to the condenser until the temperature anddensity of the steam remaining is reduced to a preselected level Wherefurther agitation will not result in enough friction heating to beharmful to the turbine components.

The special venting system includes a conduit 18 leading from opening 1dat the inlet end of turbine 1 to condenser 3. Located in conduit 18 forcontrolling the flow of steam therethrough is valve 19 which may beactuated by any suitable servo device, such as solenoid 21 in electricalcircuit 22. The closing of circuit 22 is effected by downward movementof rod 12a which has mounted thereon switch member 24 that engagescontacts 23 to close circuit 22 when control valve 7 reaches the fullyclosed position. Closing circuit 22 energizes solenoid 21 to open valve19 against the biasing action of spring 2%), which is arranged to closevalve 19 when the solenoid 21 is deenergized.

In the principal embodiment illustrated, the opening of valve 19provides a flow path through conduit 1% to the condenser for the steamcontained in conduit 8, reheater b, conduit 11, and high pressureturbine 1. For preventing excessive temperatures in the condenser, whichmight otherwise damage the heat exchange tubes therein, atemperature-reducing desuperheater is provided as shown at 17. Thedesuperheater is merely a heat exchanger in which water is sprayed toreduce the temperature of the steam.

Although all the steam trapped upstream from the intercept valve '10 maybe exhausted through valve 19, it is not desirable to do this, since,after a predetermined amount of steam is exhausted from the reheater,the steam pressure is reduced with a resultant lower steam tempera ture.If the fiow to the condenser is permitted to continue, the steamtemperature would rapidly drop to a value where the temperaturedifierence between the still hot turbine components and the coolingsteam would be sufficient to result in thermal shock, which has a mostserious effect on the :high temperature alloys employed. To preventexcessive cooling and resulting thermal stresses, valve 19 is closedwhen the steam in the reheater drops to a predetermined pressure, soselected that the density of the steam is reduced to where thetemperature rise resulting from further friction heating action of therotor, taking into account the normal heat loss by radiation etc., willnot damage the turbine. To accomplish this, a pressureoperated switch 25responsive to the pressure located in the conduit 11 is located betweenhigh pressure turbine 1 and reheater 5b. Switch 25 is biased by spring30 to move switch member 27 out of engagement with contacts 28 againstthe steam pressure in conduit 11 communicated to chamber 26. When thepressure in conduit 11 drops below the predetermined value, switch 27becomes disengaged from contacts 28 to open circuit 22 and de-energizesolenoid 21. Valve 19 is then moved by spring 26 to closed position,regardless of the position of switch 24 with respect to contacts 23.

The operation of my novel venting system can be seen from the following.

When the main control valve 7 and intercept valve are closed due to anemergency overspeed condition or a system failure, high density steam istrapped in turbine 1, conduit 11, reheater 5b, and conduit 8. Whencontrol valve 7 reaches the fully closed position, switch 24 engagescontacts 23 to close circuit 22 and energize solenoid 21 and open valve19 against the action of spring The steam trapped between control valve7 and intercept valve 10 now flows backwardly as indicated by the dottedarrows in the drawing through high pressure turbine 1, 'valve 19 andconduit '18 to the condenser 3 due to the large differential in pressurebetween turbine 1 and condenser 3. Thus, the steam in turbine 1 isexhausted to the condenser before it can be reheated by friction, due tochurning action of rotor 1b, to a higher than normal operatingtemperature. The steam is appropriately cooled by the water spray indesuperheater 17 before the steam enters condenser 3 so that thecondenser tubes will not be damaged. When the steam begins to evacuatefrom reheater 5b, the pressure and temperature of the trapped steambegins to drop and at a predetermined steam pressure, spring 3t biasespiston 29 to disengage switch 27 from contacts 28 and open circuit 22.The opening of circuit 22 de-en ergizes solenoid Z1 and valve 19 ismoved to the closed position by spring 20. This action thus prevents thecomplete evacuation of all the steam trapped between the high pressureturbine 1 and intercept valve 10 to prevent the drop in temperature ofthe steam to a point where too rapid cooling of the high pressureturbine could cause thermal shock.

In some cases it may be preferred to prevent the evacuation of steamfrom the reheater 5b and conduit 8 through bypass conduit 18. To thisend, the previously disclosed arrangement can be modified, as shown inFIG. 2 on the drawing. In place of the pressure switch 25 and conduit11, a non-return valve 31 can be located in conduit 11a extendingbetween turbine 1 and reheater 5b. The check valve 31 prevents theevacuation of steam from reheater 5b back through the turbine 1 when thebypass valve 19 is open. Thus the reheater will remain at normaloperating temperature and pressure while only the dense steam in turbine1 is vented to the condenser.

Thus it is seen that the invention provides a novel venting system whichexhausts to the condenser the steam trapped in the high temperatureturbine to prevent friction heating of the steam and turbine componentsby the rotor When the control and intercept valves are suddenly closed.

While only two embodiments have been disclosed herein, and those inquite diagrammatic fashion, it will be apparent that many changes andsubstitutions of equivalents may be made without departing from theinvention. For example, although the venting system is shown applied toa reheat steam turbine powerplant, it would obviously apply to amultiple turbine arrangement in which the steam was not reheated. Inaddition, the control valve 7 might be an intercept valve controllingthe How of steam from a reheater after expansion through a precedingsupercritical pressure turbine stage. Also, the bypass valve 19 and thecontrol therefor can obviously take the form of any of numerouspneumatic, hydraulic, or electric control devices. In addition, valve 19could be operated mechanically by a special speed governor connecteddirectly to the turbine shaft, or other mechanisms which would beapparent to one skilled in the art. If valve 19 was not electricallyoperated, switching mechanism 25 could, of course, be suitably modifiedto close valve 19 when the pressure in conduit 11 drops to a certainvalue.

It is, of course, intended to cover by the appended claims all suchmodifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In a reheat turbine powerplant having conduits connecting in seriessteam generating means, high pressure turbine, reheater, lower pressureturbine and condenser, first valve means controlling the flow of motivefluid to the high pressure turbine, second valve means controlling theflow between the reheater and lower pressure turbine, and governingmeans controlling the operation of said first and second valve means andadapted to close them upon occurrence of a predetermined operatingcondition, the combination of bypass conduit means with bypass valvemeans communicating between the high pressure turbine and condenser,electrical means including first switching means operated by thegoverning means for moving the bypass valve to the open position whenthe first and second valves are closed to exhaust the entrapped fluid tothe condenser, pressure responsive switching means in series with saidfirst switching means and exposed to the pressure in the conduitcommunicating between the high pressure turbine and reheater, saidpressure responsive switching means being constructed and arranged toactuate the electrical means to move the bypass valve to the closedposition after a predetermined amount of motive fluid has been exhaustedto the condenser.

2. In a reheat turbine powerplant having conduits connecting in seriessteam generating means, a high pressure turbine, a reheater, a lowerpressure turbine, and condenser, first valve means controlling the flowof motive fluid to the high pressure turbine, second valve meanscontrolling the flow of motive fluid from the reheater to the lowerpressure turbine, and governing means controlling the operation of thefirst and second valve means and adapted to close them upon occurrenceof a predetermined operating condition, the combination of bypassconduit means with bypass valve means communicating between the highpressure turbine and condenser, means controlled by said governing meansfor opening the bypass valve when the first and second valves areclosed, and check valve means in the conduit between the high pressureturbine and reheater, whereby the fluid trapped between the first valvemeans and the check valve means is vented to the condenser to preventoverheating of the turbine components by friction heating of the trappedfluid.

3. In governing mechanism for a reheat turbine powerplant havingconduits connecting in series steam generating means, a high pressureturbine, a reheater, a lower pressure turbine, and condenser, a mainValve controlling the flow of motive fluid to the high pressure turbine,an intercept valve controlling the flow of fluid between the reheaterand lower pressure turbine, and governing means for closing the maincontrol and intercept valves when the turbine exceeds a predeterminedspeed, the combination of conduit means including a bypass valvecommunicating between the high pressure turbine and condenser, meansactuated by said governing means to open the bypass valve when the maincontrol and intercept valves are closed, pressure responsive meansexposed to the pressure in the conduit communicating between the highpressure turbine and reheater and being constructed and arranged to movethe bypass valve to the closed position after the pressure has droppedto a predetermined level as the motive fluid is exhausted to thecondenser, and desuperheater means in the bypass conduit for cooling thebypassed steam to prevent damage to the condenser.

4. In a reheat turbine powerplant having conduits connecting in seriessteam generating means, a high pressure turbine, at reheater, a lowerpressure turbine, and condenser, first valve means controlling the flowof motive fluid to the high pressure turbine, second valve meanscontrolling the flow of motive fluid from the reheater to the lowerpressure turbine, and governing means controlling the operation of thefirst and second valve means and adapted to close them upon occurrenceof a predetermined operating condition, the combination of separatebypass conduit means with bypass valve means communicating between thehigh pressure turbine and condenser, means controlled by said governingmeans for opening the bypass valve when the first and second valves areclosed, and means to hold the pressure in the reheater to a predeteminedlevel after said bypass valve is opened, whereby the high pressure steamtrapped in the high pressure turbine is caused to flow throughthe highpressure turbine to the condenser to prevent the turbine components fromoverheating by friction heating of the trapped steam.

5. Elastic fluid turbine plant in accordancewith claim 4 where saidmeans to hold the pressure in the reheater to a predetermined levelcomprises means responsive to motive fluid pressure in the first turbineoutlet end portion for re-closing the bypass valve when the fluidpressure level in said outlet end portion falls to a preselected valueto prevent excessive cooling of said turbine.

6. Elastic fluid turbine plant in accordance with claim 4 where saidmeans to hold the pressure in the reheater to a predetermined levelcomprises non-return valve means in the conduit connecting the firstturbine outlet end por tion to the reheater for preventing reverse flowfrom the reheater into the first turbine when the vent valve opens.

7. In a high pressure elastic fluid turbine powerplant, the combinationof inlet valve means admitting high pressure motive fluid to first andsecond turbine sections connected in series with intercept valve meanscontrolling the admission of fluid to the second turbine, a vent conduitcommunicating with the inlet end portion of the first turbine, normallyclosed vent valve means in said vent conduit, and speed responsive meansfor opening said vent valve means and simultaneously closing the inletand intercept valves, whereby high pressure motive fluid trapped in thefirst turbine flows to the inlet end of the first turbine and escapesthrough the vent conduit to prevent friction heating of the trappedfluid by the churning action of the first turbine rotor, and means tohold the pressure between the intercept valve and the outlet end portionof the first turbine to a predetermined level after said vent valvemeans is opened.

References Cited in the file of this patent UNITED STATES PATENTS1,620,662 Kasley Mar. 15, 1927 1,726,561 Hodgkinson et al Sept. 3, 19291,934,667 Harter Nov. 7, 1933 2,232,852 Hemenway Feb. 25, 1941 2,586,510Bryant Feb. 19, 1952 2,747,373 Eggenberger et al May 29, 1956

2. IN A REHEAT TURBINE POWERPLANT HAVING CONDUITS CONNECTING IN SERIESSTEAM GENERATING MEANS, A HIGH PRESSURE TURBINE, A REHEATER, A LOWERPRESSURE TURBINE, AND CONDENSER, FIRST VALVE MEANS CONTROLLING THE FLOWOF MOTIVE FLUID TO THE HIGH PRESSURE TURBINE, SECOND VALVE MEANSCONTROLLING THE FLOW OF MOTIVE FLUID FROM THE REHEATER TO THE LOWERPRESSURE TURBINE, AND GOVERNING MEANS CONTROLLING THE OPERATION OF THEFIRST AND SECOND VALVE MEANS AND ADAPTED TO CLOSE THEM UPON OCCURRENCEOF A PREDETERMINED OPERATING CONDITION, THE COMBINATION OF BYPASSCONDUIT MEANS WITH BYPASS VALVE MEANS COMMUNICATING BETWEEN THE HIGHPRESSURE TURBINE AND CONDENSER, MEANS CONTROLLED BY SAID GOVERNING MEANSFOR OPENING THE BYPASS VALVE WHEN THE FIRST AND SECOND VALVES ARE CLOSEDAND CHECK VALVE MEANS IN THE CONDUIT BETWEEN THE HIGH PRESSURE TURBINEAND REHEATER, WHEREBY THE FLUID TRAPPED BETWEEN THE FIRST VALVE MEANSAND THE CHECK VALVE MEANS IS VENTED TO THE CONDENSER TO PREVENTOVERHEATING OF THE TURBINE COMPONENTS BY FRICTION HEATING OF THE TRAPPEDFLUID.